Using a controlled experimental animal model with antral vein cannulation the effect of topically applied solutions of varying pH, with and without calcium, on serum gastrin and fundic acid production will be tested. Differential effects of fundic and antral mucosa will be studied. In addition, the importance of G-Cell mass in clinical situations, particularly in stress ulceration will be studied using endoscopy, antral biopsy and lithium flux techniques. These results will be correlated with clinical progress, gastric secretion and serum gastrin. Investigation into the important trophic factors augmenting G-Cell mass will be studied in the colony of hyperparathyroid rats.